1. Field of the Invention
This invention relates to a method for efficiently separating and concentrating a useful organic component having a lower boiling point than water (hereinafter referred to as an organic component having a low boiling point) from an aqueous solution (such as a broth or the like) by the utilization of a hydrophobic porous membrane and a hydrophobic nonporous membrane.
2. Description of the Prior Art
Conventionally, the commonest method for separating and concentrating an organic component having a low boiling point from an aqueous solution containing organic materials has been distillation. Recently, studies have also been made of other separation and concentration methods including the methods of concentration by use of a reverse osmosis membrane or by pervaporation through a membrane.
Moreover, if fermentation is carried out continuously, the resulting fermentation product accumulates in the system and acts as a fermentation inhibitor. As a result, it becomes impossible to continue the fermentation. In order to avoid this difficulty, a continuous fermentation process is known in which the fermentation product is continuously separated from the broth within the fermenter by use of a hydrophilic ultrafiltration membrane.
However, in the conventional distillation method for the separation and concentration of an organic component having a low boiling point from an aqueous solution, it is necessary to distil and concentrate an aqueous solution having a relatively low concentration. This is disadvantageous in that a complicated and large-sized plant is required and large energy consumption is involved. In the reverse osmosis method, the practically achievable concentration is limited to a low level. In the case of ethanol, for example, the resulting concentrated solution has a concentration limit of about 15%. This concentration level is inadequate for practical purposes. In the pervaporation method, water is selectively allowed to penetrate through a membrane so as to bring about the resultant concentration of an organic component having a low boiling point. Accordingly, great energy loss is caused because of the removal of large amounts of water.
Moreover, in the continuous fermentation process using an ultrafiltration membrane, both water and the fermentation product having a low boiling point are separated from the fermentation system, so that the raw material for fermentation (such as glucose) and the inorganic salts also go out of the fermentation system. Thus, the fermentation product must subsequently be separated therefrom. Usually, the residue from which the fermentation product has been separated is discarded. This is not only disadvantageous from an economic point of view, but also poses a problem in that the raw material for fermentation and the inorganic salts must be supplied to the fermenter so as to make up for the loss.
In order to overcome the disadvantages of the above-described prior art methods, the present inventors carried out intensive research and found that a hydrophobic porous membrane allows the passage of vapors, but does not allow the passage of an organic material-containing aqueous solution in liquid form. On the basis of this finding, the present inventor devised a method for separating and concentrating an organic component having a low boiling point from an aqueous solution containing organic components by bringing the aqueous solution containing organic components into contact with one surface of a hydrophobic porous membrane and evacuating the side of the porous membrane opposite to the aqueous solution, and disclosed it in the previous Japanese patent application No. 39408/'85. Although this method is efficient, it has been found to have the disadvantage that, when the concentration of the organic component in the aqueous solution has increased and exceeded a certain level, the pores of the hydrophobic porous membrane are made hydrophilic. This prevents further concentration because the aqueous solution in liquid form can now pass through the membrane. For example, where the organic component having a low boiling point is ethanol, it cannot be concentrated to 70% or more for the abovedescribed reason.
Furthermore, concentration methods using a hydrophobic nonporous membrane are also known. For example, silicone rubber is more permeable to the vapors of organic components having a low boiling point (such as ethanol and the like) than to water vapor, and thus makes it possible to concentrate such organic components having a low boiling point. Since such membranes are nonporous, they do not involve the above-described problems. However, hydrophobic nonporous membranes are still disadvantageous in that they have a considerably lower permeability to vapors than porous membranes and, especially when the aqueous solution has a low organic component concentration, the above permeability is too low for practical purposes because the membrane does not swell so much due to the organic component.